1. Field of the Disclosure
The present disclosure relates to a laser system operative to output light at stabilized wavelengths and provide emitter protection from backreflected light.
2. Prior Art Discussion
There are several types of lasers, including gas lasers, solid-state lasers, liquid (dye) lasers, free electron, and semiconductor lasers. All lasers have a laser cavity defined by an optical gain medium in the laser cavity and a method for providing optical feedback. The gain medium amplifies electromagnetic waves (light) in the cavity by stimulated emission, thereby providing optical gain.
A semiconductor active region serves as the gain medium of semiconductor lasers which are used for a variety of industrial and scientific applications. In use, semiconductor lasers are attractive for forming a source of optical energy due to, among others, a space- and wall-plug efficient configuration.
Semiconductor lasers, like the rest of optoelectronic devices, have to meet very stringent requirements. As known to one of ordinary skills in the laser arts, semiconductor lasers have the inherent problem of wavelength uncertainty of output light dependence from temperature and driving current due to the large spectral width of their gain media. In certain applications, particularly those that require high powers including, among others, material processing, military and others, such a wavelength fluctuation is highly undesirable.
One well-known means for stabilizing a wavelength involves coupling an external spectrally-selective component, such as a Bragg mirror, to a gain chip at the output facet thereof. Such a configuration is known as a Distributed Bragg Reflector (DBR) laser with a Bragg reflector mirror defining an external cavity with the input faucet of the gain chip.
FIG. 1 illustrates plots of wavelengths with and without the use of the volume Bragg mirror. As can be seen, the Bragg volume mirror narrows the emission spectrum of the chip output and conditions the chip to operate at the resonant frequency of the volume Bragg mirror above the threshold of lasing.
As light is radiated by a gain element, such as a laser diode, it impinges upon a variety of barriers including a spectrally-selective unit provided with a mirror, such as volume Bragg mirror. The spectrally-selective unit is configured with piece of photosensitive material having the Bragg mirror recorded therein in accordance with techniques disclosed in patent application publications which are assigned to PD-LD, Inc, ONPAX and others and too numerous to be individually listed here. While, the reflection of light at the resonant frequency from the Bragg mirror into the internal cavity of the laser diode is essential for a stabilized operation of the gain element, other frequencies backreflected into the internal cavity are highly undesirable. Some of the light emitted by the gain element is incident upon the photosensitive material before it hits the Bragg mirror. In the configurations of the known prior art, the material reflects a portion of chip-emitted light back into the inner cavity, thereby detrimentally affecting the operation of the gain element.
The light backreflected by material hosting a Bragg mirror, also known as chirped volume Bragg grating, is not the only light that may be backreflected back into the internal cavity. A signal light generated by a laser system, pumped by the gain element, is capable of backreflecting onto the internal cavity of the gain element as well. The ever-increasing power of gain elements, such as laser diodes, satisfying the industrial demands, is associated with powerful backreflected light signals. Typically, to prevent propagation of the laser system signals into the internal cavity of the gain element, optical isolators are placed downstream from the gain element. The isolators adequately protect the internal cavity but at the increased cost and complexity of the entire laser system.
Accordingly, what is needed is a gain element having a safeguard mechanism which is operative to prevent propagation of parasitic backreflected light generated by the gain element into the internal cavity of the gain element, without, however, rendering the entire structure cost-ineffective.
A further need exists for a configuration of laser system providing for the improved and cost-effective protection of a gain element from backreflected signal light.